ipv4: Initialise ->flowi4_scope properly in ICMP handlers.

All the *_redirect() and *_update_pmtu() functions initialise their
struct flowi4 variable with either __build_flow_key() or
build_sk_flow_key(). When sk is provided, these functions use
RT_CONN_FLAGS() to set ->flowi4_tos and always use RT_SCOPE_UNIVERSE
for ->flowi4_scope. Then they rely on ip_rt_fix_tos() to adjust the
scope based on the RTO_ONLINK bit and to mask the tos with
IPTOS_RT_MASK.

This patch modifies __build_flow_key() and build_sk_flow_key() to
properly initialise ->flowi4_tos and ->flowi4_scope, so that the
ICMP redirects and PMTU handlers don't need an extra call to
ip_rt_fix_tos() before doing a fib lookup. That is, we:

  * Drop RT_CONN_FLAGS(): use ip_sock_rt_tos() and ip_sock_rt_scope()
    instead, so that we don't have to rely on ip_rt_fix_tos() to adjust
    the scope anymore.

  * Apply IPTOS_RT_MASK to the tos, so that we don't need
    ip_rt_fix_tos() to do it for us.

  * Drop the ip_rt_fix_tos() calls that now become useless.

The only remaining ip_rt_fix_tos() caller is ip_route_output_key_hash()
which needs it as long as external callers still use the RTO_ONLINK
flag.

Note:
  This patch also drops some useless RT_TOS() calls as IPTOS_RT_MASK is
  a stronger mask.

Signed-off-by: Guillaume Nault <gnault@redhat.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>